Such actuators are used, for instance, in the field of motor vehicle technology for operating clutches in manual or automatic transmissions, dampers, or brakes etc. In manual transmissions, the actuator usually consists of a hydraulic system that is directly operated by the driver. A hydraulic section of a specific transmission ratio transmits the operating energy from a pedal that has been operated by the driver to a disengagement element of the clutch. In automatic transmissions or hybrid drive trains, there are a number of different operating devices or actuators that are implemented in the form of combinations of hydraulic, mechanical, or electrical systems.
A known use for magnetorheological fluids (also abbreviated to MRF in the following text) is in dampers, brakes, and clutches. A magnetorheological fluid (MRF) is a suspension of magnetically polarizable particles (such as carbonyl iron powder) that are finely distributed in a carrier fluid. In such known applications, the MRF is used as a direct clutch/damping/braking fluid. Main disadvantages of the indicated systems when they are integrated in compact clutches are size, complexity, and/or costs. In particular in fast-rotating applications, a disadvantage of known MRF clutches is that a complete separation as it is possible in conventional clutches of the strands that are connected by the clutch is impossible because there will always be residual friction in the clutch due to the MRF, a fact which is detrimental to the degree of efficiency. Another disadvantage is that the centrifugal effect on the particles of the MRF may affect the torque behavior.
EP 1 438 517 B1 for instance discloses a controllable brake comprising a) a rotor embodied to have a working portion on its periphery which extends parallel to a shaft on which said rotor is mounted, b) a shaft on which said rotor is mounted in a manner to restrain a relative movement between the shaft and the rotor, c) a housing having a first chamber rotatably housing the rotor therein and including a magnetic field generator spaced apart and separate from the rotor and configured and positioned to generate magnetic flux through a magnetically controllable material in a direction perpendicular to the shaft and to the working portion of the rotor, wherein d) the magnetically controllable material contained within said first chamber is in contact with at least the working portion of the rotor, and wherein the brake is characterized by e) an active return-to-center device in the first chamber to cause the rotor to return to a relative center position
EP 2 060 800B1 discloses a combined actuator comprising a mobile element, a drive for displacing the mobile element along a path, and a rheological control brake mechanically coupled to the movable element to adjust the displacement of the movable element along the path, wherein the combined actuator is characterized in that the rheological control brake comprises at least two contiguous readjustment chambers, a magnetorheological fluid contained in the two readjustment chambers, at least one sliding piston, which tightly separates the two adjustment chambers and is mechanically connected to the mobile element, at least one external connection pipe, which reciprocally connects the two adjustment chambers and is arranged outside the two adjustment chambers themselves, and a driving device which is coupled to the external connection pipe so as to apply a variable magnetic field to the rheological fluid contained in the exterior connection pipe for varying the viscosity of the rheological fluid between a minimum value, at which the rheological fluid may freely flow through the exterior connection pipe, and a maximum value, at which the rheological fluid cannot flow through the external connection pipe, wherein the driving device comprises a ferromagnetic core in the form of an open ring having an interruption defining a gap in which the exterior connection pipe is arranged, at least one winding which is coupled to the ferromagnetic core, and an electric generator connected to the winding to circulate an electric current of adjustable intensity in the winding.
WO 00/53936 A1 discloses a controllable pneumatic apparatus comprising a pneumatic system including a pneumatic actuator having a housing, a piston disposed in the housing and movable responsive to a pressure differential acting upon the piston, and at least one output member coupled to the piston, and a rotary-acting controllable brake including a field responsive medium contained therein, the rotary-acting controllable brake including a brake shaft coupled to the output member for controlling motion thereof.